1. Field of the Invention
The invention relates to an exhaust gas purification catalyst including a composite oxide support and a precious metal catalyst supported on a surface thereof.
2. Description of Related Art
In various kinds of industrial fields, various efforts for reducing an environmental impact are being forwarded worldwide. In particular, in an automobile industry, a development for popularization and further performance improvement of not only gasoline engine cars excellent in the fuel consumption performance but also of so-called ecologically-friendly cars such as hybrid cars and electric cars is being advanced daily. In addition to the development of such the ecologically-friendly cars, a study on exhaust gas purification catalysts for purifying an exhaust gas discharged from an engine has been actively forwarded. The exhaust gas purification catalyst includes an oxide catalyst, a ternary catalyst, and a NOx storage and reduction catalyst. In the exhaust gas purification catalyst, a catalyst of a precious metal such as platinum, rhodium, or palladium develops catalyst activity. The precious metal catalyst is generally used in a state supported on a support composed of porous oxide such as alumina.
The exhaust gas purification catalyst is generally used under a high temperature atmosphere. Accordingly, it is desirable that a catalyst has high heat resistance that allows to maintain high catalyst activity even after long term use under a high temperature atmosphere.
Now, in Japanese Patent Application Publication No. 2009-107901 (JP 2009-107901 A), a particulate inorganic mixed oxide that contains aluminum, zirconium, cerium and first and second additive elements is disclosed. The first and second additive elements are selected from the group consisting of rare earth elements excluding cerium and alkali earth elements. More specifically, the inorganic mixed oxide is composed as shown below. A rate of content of aluminum in the inorganic mixed oxide is 60 to 90% by atom as an element with respect to a total amount of elements that become a positive ion in the inorganic mixed oxide; a rate of content of cerium in the inorganic mixed oxide is 0.4 to 50% by atom as an element with respect to a total amount of zirconium and cerium in the inorganic mixed oxide; a rate of content of a total amount of the first and second additive elements is 1 to 12% by atom as elements with respect to a total amount of elements that become positive ions in the inorganic mixed oxide; 80% or more of primary particles of the inorganic mixed oxide has a particle size of 100 nm or less; and in at least a part of superficial portion of the primary particles, a surface concentration region where a rate of content of the second additive element is locally enhanced is formed. Preferably, the first additive element is lanthanum and the second additive element is neodymium.
An inorganic mixed oxide where neodymium oxide is concentrated on a surface of oxides of aluminum, zirconium and cerium, that is, alumina, zirconia and ceria is a raw material excellent in the heat resistance. Further, the inorganic mixed oxide exerts oxygen storage capacity (OSC capacity), HC modification capacity and NOx purification performance with good balance.